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Register a new userElectron pairing in the presence of incipient bands in iron-based superconductors
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Recent experiments on certain Fe-based superconductors have hinted at a role for paired electrons in incipient bands that are close to, but do not cross the Fermi level. Related theoretical works disagree on whether or not strong-coupling superconductivity is required to explain such effects, and whether a critical interaction strength exists. In this work, we consider vario
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Electron correlations play a central role in iron-based superconductors. In these systems, multiple Fe $3d$-orbitals are active in the low-energy physics, and they are not all degenerate. For these reasons, the role of orbital-selective correlations has been an active topic in the study of the iron-based systems. In this paper, we survey the recent developments on the subject. For the normal state, we emphasize the orbital-selective Mott physics that has been extensively studied, especially in the iron chalcogenides, in the case of electron filling $n sim 6$. In addition, the interplay between orbital selectivity and electronic nematicity is addressed. For the superconducting state, we summarize the initial ideas for orbital-selective pairing, and discuss the recent explosive activities along this direction. We close with some perspectives on several emerging topics. These include the evolution of the orbital-selective correlations, magnetic and nematic orders and superconductivity as the electron filling factor is reduced from $6$ to $5$, as well as the interplay between electron correlations and topological bandstructure in iron-based superconductors.
We theoretically study the spin fluctuation and superconductivity in La1111 and Sm1111 iron-based superconductors for a wide range of electron doping. When we take into account the band structure variation by electron doping, the hole Fermi surface originating from the $d_{X^2-Y^2}$ orbital turns out to be robust against electron doping, and this gives rise to large spin fluctuations and consequently $spm$ pairing even in the heavily doped regime. The stable hole Fermi surface is larger for Sm1111 than for La1111, which can be considered as the origin of the apparent difference in the phase diagram.
Motivated by a recent experiment of Song emph{et al.} [Science {bf 332}, 1410 (2011)], we theoretically study the spin dynamics, charge dynamics, and point-contact Andreev-reflection spectroscopy (PCARS) of two-band iron-based superconductors of a possible extended $s_pm$-wave pairing symmetry. We consider the case of a dominant $s_pm$ gap blended by a secondary extended $s$ component in which gap nodes can develop in the Fermi pockets near zone corner and/or boundary. Due to the strong nesting effect associated with nodal regions, dynamical spin and charge susceptibilities can exhibit strong peaks at momenta near $(pmpi/2,0)$, $(pmpi,pmpi/2)$, as well as $(pmpi,0)$ in the unfolded Brillouin zone. For PCARS, considering an anisotropic band effect induced by an applied voltage, [100] differential conductance can exhibit a $V$-shape behavior manifesting a gap node occurring in such direction. It is highly suggested that the above features can be experimentally investigated to help sorting out the pairing symmetry of iron-based superconductors.
We use magnetic long range order as a tool to probe the Cooper pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron diffraction experiments, in particular for the unusual behavior of the magnetic order below $T_{c}$, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.
The discovery of high-temperature superconductivity in iron pnictides raised the possibility of an unconventional superconducting mechanism in multiband materials. The observation of Fermi-surface(FS)-dependent nodeless superconducting gaps suggested that inter-FS interactions may play a crucial role in superconducting pairing. In the optimally hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, the pairing strength is enhanced simultaneously (2$Delta$/Tc$sim$7) on the nearly nested FS pockets, i.e. the inner holelike ($alpha$) FS and the two hybridized electronlike FSs, while the pairing remains weak (2$Delta$/Tc$sim$3.6) in the poorly-nested outer hole-like ($beta$) FS. Here we report that in the electron-doped BaFe$_{1.85}$Co$_{0.15}$As$_2$ the FS nesting condition switches from the $alpha$ to the $beta$ FS due to the opposite size changes for hole- and electron-like FSs upon electron doping. The strong pairing strength (2$Delta$/Tc$sim$6) is also found to switch to the nested $beta$ FS, indicating an intimate connection between FS nesting and superconducting pairing, and strongly supporting the inter-FS pairing mechanism in the iron-based superconductors.
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